1. Field of the Invention
This invention relates to a method of drying moist skins, and to apparatus for putting this method into practice.
By the term "skins" there is to be understood both raw or tanned animal skins as well as leathers of all types.
2. Description of the Prior Art
The drying of initially pre-stressed moist skins is known and has been effected over a long period by simply nailing skins on frames or panels of wood by two operators disposed opposite to one another and pulling simultaneously the edge of the skin at two opposed points on its periphery before placing a nail in the said edges adjacent to the pulling points, and this operation is repeated around the periphery of the skin. The skins then dry by natural ventilation or in drying rooms where the nailing frames are placed.
This rudimentary method is slow and laborious, the tensioning of the skin is irregular and the periphery of the skin is damaged by the holes made by the nails.
In order to reduce these disadvantages there have been proposed subsequently, metal perforated frames on which the skins are stretched in all directions manually by menas of wedge clamps provided with a catch at their lower part, the said catch being engaged in one of the holes of the perforated frames, which are placed in the drying rooms utilising hot air.
But the stretching of the skins, remaining independent of the force exerted by the operators, is irregular and the overall operations remain laborious and slow.
In order to equalize the tension exerted on the skins, to render the operation less laborious and improve productivity, an apparatus has been proposed in which the metal perforated frames are divided into two parts which become spaced from one another after the operators have secured the skins, without stretching them. This spacing is effected either by purely mechanical means (cams and guides) over a predetermined distance, or by pneumatic or hydraulic means using a predetermined force. The frames, either individual or forming a continuous surface, then pass into ventilated hot air enclosures where the stretched skins are exposed to infra-red radiation for drying purposes.
These apparatus have the serious disadvantage of only stretching the skin in a single sense and not in all directions as is necessary because of the irregular shape of the skins, and as it has been previously effected, although imperfectly, by the previous manual stretching.
Another disadvantage of these apparatus is that for predetermined control the two halves of each skin are neither spaced by a fixed distance, nor stretched with a fixed force without having regard to the dimension of the skin between the two opposed clamps, nor to the variable dimensions of the different skins, in such a manner that the parts of the skin of small dimension are stretched through the same distance or with the same force as the larger parts, which is clearly illogical and causes a loss of surface following the stretching in a single sense, and distortions which affect the structure and the properties of the skins for their final uses.
Finally, apparatus are known which effect stretching of the skins in multiple directions, the stretching clamps being mounted in slides placed as the spokes of a wheel within the interior of a stretching frame, stretching being effected in accordance with a predetermined force, identical for all the clamps, by pneumatic or hydraulic means. As hitherto known, the stretching frames are placed in a hot air ventilating room.
The disadvantage of stretching in accordance with a fixed force, without regard to the dimension of the skin between the two opposed clamps or to the disparitities in the dimensions of the skins therefore remains.
Another disadvantage of these apparatus is their extreme complexity in order to effect multiple movements on a large number of individual clamps, which gives rise to many risks in operation and an excessively high price which takes them beyond any economic justification, two reasons for which the use of these apparatus has not been developed.
In addition, all the apparatus referred to hereinbefore give rise to other disadvantages with regard to the drying itself. The hot air ventilation drying rooms dry slowly because of the low temperature needed for a good quality of final skins, which necessitates a fairly large number of stretching frames for a given production, with as a consequence, very bulky and costly installations. Moreover, drying by infra-red radiation used to reduce the duration of the drying time and the importance of installations applied to skins at an elevated temperature seriously affects their quality. In addition, the energy efficiency of short wave radiations used is low.